[ARC5] R-4

[Mike Morrow]

w8fdv asked:

> What is an R-4?

I'm assuming that you mean R-4(*)/ARR-2.

It's the airborne part of a WWII and 1950s USN aircraft homing system.

The system operated from 234 to 258 mcs (typically 246 mc), and a typical aircraft carrier or base transmitter system might have been a "YG."  The YG had a narrow beam rotating antenna system and transmitted a different Morse character depending on which way the antenna was oriented and the ship was headed.  The pilot had decode cards which told him which true radial he was on for the code character he was receiving.  

The cute thing about this system is that the transmitted 246 mcs signal was modulated at one of six different frequencies in the AM broadcast band (from 540 to 1030 kc).  Assuming that the modulation channel selected was 830 kc, the TRF front end and grid-leak detector demodulated the 830 kc modulation on the 246 mc signal, then the second half of the receiver detected the 830 kc signal and its BFO produced a Morse beat note as the 830 kc modulating signal was keyed at the transmitter.  Voice ID and other info could also be transmitted.  An enemy that intercepted the the 246 mc signal could hear nothing of the 840 kc signal in his headphones.  

Dial calibration marks leave off the "2" so you'll actually see "46" rather than "246."

The AN/ARR-2 replaced the earlier ZB-series and R-1/ARR-1 homing adapters which were just a front end which fed into a separate receiver tuned to the appropriate modulating frequency in the broadcast band (RU, RAX, ARB, R-24/ARC-5, BC-946 for USAAF, etc.).

It's a neat system, developed before WWII.  Most of the early ZB-series units actually show all three digits on the tuning dial.

On the R-4/ARR-2, the small tuning dial on the lower left corner sets the front end frequency, and the tuning spline control changes a number 1 to 6 in the window to set the second part of the receiver to one of the six modulating frequencies.

The receiver is designed to fit into an ARC-5 receiver rack.  A late-war Navy single-place fighter might typically have had an R-4/ARR-2 homing receiver, a fixed-tuned R-26/ARC-5 HF communications receiver, and a R-28/ARC-5 four-channel VHF communcations receiver in the fighter's three-receiver ARC-5 command radio rack.  The control boxes for the communications gear in that system would be a C-30A and C-38/ARC-5.  The C-38/ARC-5 has all the controls needed for the R-4/ARR-2.  I think of this system as sort of an early example of integrated avionics.

I have AN/ARR-2 manuals whose revision dates indicate that the system was used into the mid-1950s.

These sets are pretty useless outside their original application, but they represent a lot of history.  Hacking one is discouraged!

73,
Mike / KK5F